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Tuesday, 31 May 2016

Why Antarctica may be the last place to feel climate change

Deep,
old water explains why Antarctic Ocean hasn't warmed

The
water around Antarctica has not seen the atmosphere for centuries,
since long before the machine age. New observations and model
simulations suggest this may be the last place on Earth to feel
climate change.

Observed
warming over the past 50 years (in degrees Celsius per decade) shows
rapid warming in the Arctic, while the Southern Ocean around
Antarctica has warmed little, if at all.

The
waters surrounding Antarctica may be one of the last places to
experience human-driven climate change. New research from the
University of Washington and the Massachusetts Institute of
Technology finds that ocean currents explain why the seawater has
stayed at roughly the same temperature while most of the rest of the
planet has warmed.

The
study resolves a scientific conundrum, and an inconsistent pattern
of warming often seized on by climate deniers. Observations and
climate models show that the unique currents around Antarctica
continually pull deep, centuries-old water up to the surface --
seawater that last touched Earth's atmosphere before the machine
age, and has never experienced fossil fuel-related climate change.
The paper is published May 30 in Nature
Geoscience.

"With
rising carbon dioxide you would expect more warming at both poles,
but we only see it at one of the poles, so something else must be
going on," said lead author Kyle Armour, a UW assistant
professor of oceanography and of atmospheric sciences. "We show
that it's for really simple reasons, and ocean currents are the hero
here."

Gale-force
westerly winds that constantly whip around Antarctica act to push
surface water north, continually drawing up water from below. The
Southern Ocean's water comes from such great depths, and from
sources that are so distant, that it will take centuries before the
water reaching the surface has experienced modern global warming.

Other
places in the oceans, like the west coast of the Americas and the
equator, draw seawater up from a few hundred meters depth, but that
doesn't have the same effect.

"The
Southern Ocean is unique because it's bringing water up from several
thousand meters [as much as 2 miles]," Armour said. "It's
really deep, old water that's coming up to the surface, all around
the continent. You have a lot of water coming to the surface, and
that water hasn't seen the atmosphere for hundreds of years."

The
water surfacing off Antarctica last saw Earth's atmosphere centuries
ago in the North Atlantic, then sank and followed circuitous paths
through the world's oceans before resurfacing off Antarctica,
hundreds or even a thousand years later.

Delayed
warming of the Antarctic Ocean is commonly seen in global climate
models. But the culprit had been wrongly identified as churning,
frigid seas mixing extra heat downward. The study used data from
Argo observational floats and other instruments to trace the path of
the missing heat.

"The
old idea was that heat taken up at the surface would just mix
downward, and that's the reason for the slow warming," Armour
said. "But the observations show that heat is actually being
carried away from Antarctica, northward along the surface."

In
the Atlantic, the northward flow of the ocean's surface continues
all the way to the Arctic. The study used dyes in model simulations
to show that seawater that has experienced the most climate change
tends to clump up around the North Pole. This is another reason why
the Arctic's ocean and sea ice are bearing the brunt of global
warming, while Antarctica is largely oblivious.

"The
oceans are acting to enhance warming in the Arctic while damping
warming around Antarctica," Armour said. "You can't
directly compare warming at the poles, because it's occurring on top
of very different ocean circulations."

Knowing
where the extra heat trapped by greenhouse gases goes, and
identifying why the poles are warming at different rates, will help
to better predict temperatures in the future.

"When
we hear the term 'global warming,' we think of warming everywhere at
the same rate," Armour said. "We are moving away from this
idea of global warming and more toward the idea of regional patterns
of warming, which are strongly shaped by ocean currents."

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Source:

The
above post is reprinted from materials provided
by University
of Washington.
The original item was written by Hannah Hickey. Note:
Materials may be edited for content and length.